Structural bonds made with adhesives are used in various industries to fasten parts or structural elements together, or to rework structures. In the aircraft industry, for example, composite patches may be attached to exterior airframe skins in order to rework and/or strengthen sections of the skin. These patches may be adhesively bonded to the skin, however, another means may require the use of mechanical fasteners to attach the patch to the skin in addition to the adhesive in order to provide secondary load paths. In the past, adhesively bonded patches with fasteners may not be certified by certifying authorities because of less than desirable predictability of the strength and durability of the bonds.
The quality and durability of an adhesive bond joint may depend upon characteristics of the bond surface to which the adhesive is applied. The surface preparation required to achieve a good bond joint may be dependent upon tight process controls and the skill of the technicians who prepare the surface. Individual surface characterization techniques are available that provide information on a single surface variable, such as surface roughness or active functional groups using profilometry or X-ray photoelectron spectroscopy (XPS), respectively. However, both profilometry and XPS typically require measurements that require the parts to be transported to a measurement unit, rendering the inspection process impractical in the field, or in a manufacturing environment, due to the logistics of transporting the parts, and/or the time required to do so. None of the presently known techniques may be used to quantify and certify the condition of bond surfaces prior to the bonding process at a level that would aid in allowing confident prediction of long-term durability of the bond joint after it enters service.
Accordingly, there is a need for a method of determining bond surface condition that may be used to aid in predicting bond joint quality, particularly in composite structures. There is also a need for a method of determining bond surface condition that may be used in the field or in a production environment and which may aid in certifying of structural bonds joints that do not employ secondary mechanical fasteners. There is a further need for a method of determining bond surface condition using measured characteristics of a prepared bond surface which take into account potentially wide variations of the measured results on the bond surface.